Flowing H 2 gas concentration was measured for the first time by using ultrasound. H2 was flowing inside a pipe and the concentration was measured from exterior of the pipe without extracting the gas from the pipe. The concentration measurement was based on speed variation of the ultrasound travelling inside the pipe through the gases. To measure the H 2 concentration drilling a hole in the pipe to extract the gases is not necessary. In this study, concentration of the H 2 gas in a flowing air was measured accurately inside the pipe. It was found that it is possible to measure the H 2 gas concentration variation in more than 50 m/s of the airflow. The response time of measuring the gas concentration was less than 0.1 s.